Positive displacement pumps, such as gear-tooth rotary pumps are well known in the art. Such pumps are generally used for the transport of fluid substances such as more and less viscous liquids, pastes and suspensions. Positive displacement pumps usually comprise at least two rotors, also referred to as pumping wheels, pumping gears and impellers.
GB patent No. 583,940 discloses a positive displacement pump comprising pumping wheels having elastic teeth or teeth with a cover layer of an elastic material. The core part of the wheel is made from a rigid material, such as a metal or a synthetic resin. The cover layer is shaped as an envelope for the teeth of the pumping wheel and is applied by endwise sliding it onto the tooth stubs. An end disc of harder material is applied for providing axial sealing on each side of the wheels. The cover layer has shown to have a relatively short durability, in particular if the pumped substance has a relatively high viscosity. Furthermore the pump has shown to have a relatively high power consumption, which among other things can be attributed to a high friction between the resilient cover material and the pumping house.
U.S. Pat. No. 2,848,952 discloses another pump comprising impellers comprising a metallic hub and a plastic cover covering the end faces of the impellers and comprising a marginal boss. The pump is arranged so that the pumped medium simultaneously is acting as lubricant between the impellers and the casing (pumping house).